Oscillators are used in various kinds of semiconductor devices. A semiconductor memory device will be used herein as an example of a semiconductor device that can use an oscillator. The semiconductor memory device may include a boost voltage generating circuit. The boost voltage generating circuit generates a boost voltage higher than an external power source voltage. The boost voltage generating circuit may be used for a word line driver, a bit line isolation circuit, and/or a data output buffer in the semiconductor memory device.
Moreover, a flash memory device may be used in portable digital devices such as a camcorder, a digital camera, a personal digital assistant (PDA), and/or an MPEG-1 layer 3 (MP3) player. A memory cell of the flash memory device may be programmed by hot electron injection and may be reset by Fowler-Nordheim (FN) tunneling occurring in an insulating layer between a source electrode of the memory cell and a floating gate of the memory cell. As such, the flash memory device may use a high voltage when programming and resetting. Thus, a flash memory device may also include a high voltage generating circuit.
A boost voltage generating circuit of a semiconductor memory device and/or a high voltage generating circuit of a flash memory device may generally include a pumping circuit and an oscillator. The pumping circuit may generate a boost voltage or a high voltage in response to a pulse control signal, and the oscillator generates a pulse control signal.
With the increasing demand for portable digital products capable of operating for a long time with a battery, attempts have been made to reduce power dissipation in portable digital products. One cause of power dissipation of portable digital products is the high voltage generating circuit of the flash memory device. Particularly, in the high voltage generating circuit, a change in a frequency of an output signal of an oscillator that controls the operation of the pumping circuit may have an influence upon the amount of current consumed by the portable digital device. Thus, it may be desirable to control the frequency of the output signal of the oscillator, so that the current consumed by the portable digital device may be reduced.
FIG. 1A illustrates a conventional ring oscillator and FIG. 1B is a detailed circuit diagram of an inverter of FIG. 1A.
Referring to FIG. 1A, a ring oscillator 10 includes a plurality, for example, first through fifth inverters 11–15 that are connected in series. A clock signal CLK_IN is input to the first inverter 11. A clock signal CLK_OUT output from the fifth inverter 15 is fed back to the first inverter 11. The clock signal CLK_OUT is delayed for a predetermined amount of time by each of the first through fifth inverters 11–15. Since the clock signal CLK_OUT output from the fifth inverter 15 is fed back to the first inverter 11, the ring oscillator 10 repeatedly outputs the clock signal CLK_OUT.
Each of the first through fifth inverters 11–15 may be implemented as a CMOS inverter. For example, referring to FIG. 1B, the first inverter 11 includes a PMOS transistor PM and an NMOS transistor NM. A source of the PMOS transistor PM is connected to a power source voltage VDD, and a drain of the PMOS transistor PM is connected to a drain of the NMOS transistor NM. A source of the NMOS transistor NM is connected to ground voltage VSS. An input signal IN is input to gates of the PMOS transistor PM and the NMOS transistor NM. An output signal OUT is output from the drains of the PMOS transistor PM and the NMOS transistor NM. The first inverter 11 delays the input signal IN for a predetermined amount of time and outputs the delayed signal as the output signal OUT.
As the power source voltage VDD decreases, the predetermined delay of the first inverter 11 increases. As a result, when the power source voltage VDD increases, the frequency of the clock signal CLK_OUT output from the ring oscillator 10 increases and when the power source voltage VDD decreases, the frequency of the clock signal CLK_OUT output from the ring oscillator 10 decreases.
FIG. 2 is a graph illustrating a relationship among a frequency of an output signal, current, and a power source voltage in a conventional oscillator included in a system. Referring to FIG. 2, graph A represents the frequency of the output signal of the oscillator with respect to the power source voltage, graph B represents current consumed by the system including the oscillator with respect to the power source voltage and the frequency of the output signal of the oscillator, and graph C represents current generated by a current generating block including the oscillator in the system with respect to the power source voltage and the frequency of the output signal of the oscillator. The current generating block may be, for example, the high voltage generating circuit of the flash memory device.
When the power source voltage increases from V1 to V2, it can be seen from graph A that the frequency of the output signal of the oscillator increases from F1 to F2. When the power source voltage increases from V1 to V2 and the frequency of the output signal of the oscillator increases from F1 to F2, it can be seen from graph B that current consumed increases from I3 to I4 and it can be seen from graph C that the current generated increases from I1 to I2. Here, I1 is the minimum amount of current for the operation of the whole system and I3 is the minimum amount of current consumed during the operation of the whole system. It can be seen from the curves of FIG. 2 that as the frequency of the output signal of the oscillator increases, the current generated and consumed by the system may also increase.
U.S. Pat. No. 6,295,217 to Yang et al., entitled “Low Power Dissipation Power Supply and Controller” describes a power supply that includes a voltage-controlled oscillator (VCO) that is responsive to rectified DC line voltage that provides for variable frequency operation in standby mode. The frequency output of the VCO is inversely proportional to the line voltage—as line voltage increases, the switching frequency decreases, which decreases the output power. See the Abstract of the Yang et al.